Power device with solderable front metal

ABSTRACT

Some exemplary embodiments of a III-nitride power device including a HEMT with multiple interconnect metal layers and a solderable front metal structure using solder bars for external circuit connections have been disclosed. The solderable front metal structure may comprise a tri-metal such as TiNiAg, and may be configured to expose source and drain contacts of the HEMT as alternating elongated digits or bars. Additionally, a single package may integrate multiple such HEMTs wherein the front metal structures expose alternating interdigitated source and drain contacts, which may be advantageous for DC-DC power conversion circuit designs using III-nitride devices. By using solder bars for external circuit connections, lateral conduction is enabled, thereby advantageously reducing device Rdson.

RELATED APPLICATIONS

This is a continuation of application Ser. No. 13/018,780 filed Feb. 1,2011.

The present application claims the benefit of and priority to aprovisional application entitled “III-Nitride Power Device HavingSolderable Front Metal with Source and Drain Solder Bars,” Ser. No.61/337,924 filed on Feb. 16, 2010. The disclosure in that provisionalapplication is hereby incorporated fully by reference into the presentapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of semiconductordevices and more particularly to the packaging of semiconductor devices.

2. Background Art

III-nitride power devices such as Gallium Nitride high electron mobilitytransistors or GaN HEMTs are known in the art for high powerperformance. However, the performance of such devices has beenconstrained by the use of conventional interconnect schemes. Thus, GaNHEMT devices utilizing multiple interconnect metal layers have beendeveloped in the art.

Such multi-level III-nitride power devices have used conventionalintegration methods, such as solder ball arrays, for integration onto asupport surface such as a printed circuit board. In particular, whenintegrating reduced footprint packages, it is often expedient to usesolder ball arrays to prevent problems such as solder bridging.Unfortunately, such conventional integration methods increase the Rdson,or the “on resistance,” of the device by limiting the flow of currentthrough the circuit, thereby negating the advantages of usingmulti-level III-nitride power devices in the first instance.

Thus, a solution is needed for integrating multi-level III-nitride powerdevices onto support surfaces while reducing Rdson, thereby leveragingthe high power performance capabilities of III-nitride power devices.

SUMMARY OF THE INVENTION

A III-nitride power device with solderable front metal, substantially asshown in and/or described in connection with at least one of thefigures, as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a multi-level III-nitridepower device according to an embodiment of the invention.

FIG. 2 illustrates a top view of a multi-level III-nitride power deviceaccording to an embodiment of the invention.

FIG. 3 illustrates a top view of a substrate for receiving a III-nitridepower device according to an embodiment of the invention.

FIG. 4 illustrates a cross-sectional view of a multi-level III-nitridepower device mounted on a substrate according to an embodiment of theinvention.

FIG. 5 illustrates a circuit diagram integrating a multi-levelIII-nitride power device according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a III-nitride power device withsolderable front metal. The following description contains specificinformation pertaining to the implementation of the present invention.One skilled in the art will recognize that the present invention may beimplemented in a manner different from that specifically discussed inthe present application. Moreover, some of the specific details of theinvention are not discussed in order not to obscure the invention. Thespecific details not described in the present application are within theknowledge of a person of ordinary skill in the art.

The drawings in the present application and their accompanying detaileddescription are directed to merely exemplary embodiments of theinvention. Further, to promote clarity and readability, drawings may notbe shown exactly to scale. To maintain brevity, other embodiments of thepresent invention are not specifically described in the presentapplication and are not specifically illustrated by the presentdrawings.

Referring to FIG. 1, a cross-sectional view of an exemplary multi-levelIII-nitride power device according to one embodiment of the invention isillustrated. In FIG. 1, power device 100 can comprise a multi-levelAlGaN/GaN High Electron Mobility Transistor (HEMT) or HeterojunctionField Effect Transistor (HFET). As illustrated in FIG. 1, power device100 includes, among other elements not shown, passivation 110,solderable front metals (SFM) 140 a and 140 b, metal layers 130 a, 130b, 130 c, 130 d and 130 e, intermetal dielectric (IMD) layers 112 a, 112b and 112 c, Tetraethooxysilane (TEOS) layers 114 a and 114 b, vias 132a, 132 b, 132 c and 132 d, Aluminum Gallium Nitride (AlGaN) layer 124,Gallium Nitride (GaN) layer 122, Silicon substrate 120, field nitride126, and gate nitride 128.

In FIG. 1, metal layer 130 a may comprise, for example, titaniumnitride, or another suitable gate metal. Metal layer 130 e may comprise,for example, aluminum, or any other suitable metal. IMD layers 112 a-112c may comprise, for example, TEOS. Passivation 110 may comprise, forexample, epoxy, polyimide, silicon oxide, or another suitable material.Epoxy may be preferable to provide sufficient thickness, such as 20 to25 microns, for maintaining structural integrity when soldering the die.As shown in FIG. 1, passivation 110 encloses and extends around SFM 140a and 140 b, for example by about five microns. In one embodiment,passivation 110 may be configured to leave a soldering surface ofapproximately 200 microns wide for matching to the width of SFM 140 aand 140 b. SFM 140 a and 140 b, as the name suggests, are configuredsuch that the top or front exposed metal is readily solderable. Forexample, SFM 140 a and 140 b may each form a tri-metal with metal layer130 e in a stacked configuration, suitable for use with high-lead soldersuch as 92.5% lead solder. One such tri-metal may comprise TiNiAg, wheremetal layer 130 e comprises aluminum, and SFM 140 a and 140 b eachcomprise Titanium as a bottom layer, Nickel as the middle layer, andSilver as a top layer. The Titanium of the tri-metal SFM is effective asa barrier layer interfacing with aluminum metal layer 130 e, while theSilver dissolves into ions to provide increased conductivity whensoldering, leaving Nickel as the contact layer.

As shown in FIG. 1, AlGaN layer 124 and GaN layer 122 are disposed ontop of a Silicon substrate 120 to form a high mobility two-dimensionalelectronic gas (2DEG). GaN based III-nitride power devices areadvantageous due to, for example, inherent high break-downcharacteristics particularly suited for power management applicationssuch as DC-DC converters. FIG. 1 shows power device 100 configured as afour interconnect metal layer power device, but alternative embodimentsmay comprise a power device with a different number of interconnectmetal layers. In the particular cross section shown in FIG. 1, vias 132a and 132 b can seen connecting metal layer 130 b to metal layer 130 c,via 132 c can be seen connecting metal layer 130 c to metal layer 130 d,and via 132 d can be seen connecting metal layer 130 d to metal layer130 e. While it may appear that insufficient vias are provided forproper connectivity, if the cross section of FIG. 1 were to be taken ata different depth, additional vias would be made visible to provide thenecessary connections to fully connect the gate, source, and drainconnections of the III-nitride device externally or to the outsideworld. By using the multi-interconnect metal layer structure and the SFM140 a and 140 b of III-nitride power device 100 shown in FIG. 1,efficient movement of current from source to drain is enabled, therebyadvantageously minimizing on resistance (Rdson), which can be criticaldue to high currents passing through the source and drain of III-nitridepower device 100.

Referring now to FIG. 2, a top view of an exemplary multi-levelIII-nitride power device in accordance with one embodiment of thepresent invention is shown. In many power applications, it may bedesirable to integrate several HFETs (or HEMTs) onto a single die. Thus,as shown in FIG. 2, power device 200 integrates two HFETs, a control FET230 and a synch FET 240. In this example, control FET 230 includes gatedigits 236, source digits 232 a, 232 b, 232 c and 232 d, and draindigits 234 a, 234 b and 234 e. Synch FET 240 includes gate digit 246,source digits 222, 242 a, 242 b and 242 c, and drain digits 244 a, 244b, 244 c and 244 d. As shown in FIG. 2, power device 200 exposes thespaced alternating interdigitated source and drain digits from the SFMson top of the die. These elongated digits may then be soldered to coppertraces on a support board using long solder bars. In this fashion,soldering surface area is optimized relative to using conventionalsolder balls or conventional solder bumps, while minimizing solderbridging. Although eight columns of digits are shown in FIG. 2,alternative embodiments may use more or fewer digits according toapplication requirements.

Referring now to FIG. 3, a top view of an exemplary substrate forreceiving the exemplary III-nitride power device in accordance with oneembodiment of the present invention is shown. As shown in FIG. 3,substrate 300 includes a cross-section A-A which can correspond to thecross-section shown in FIG. 4, discussed below. Substrate 300 mayreceive several components including III-nitride die 310 correspondingto power device 200 of FIG. 2, gate driver integrated circuit (IC) 312,and capacitors 314 a, 314 b, 314 c and 314 d. Pin 1 connection of gatedriver IC 312 is indicated by solder pad 301, with pins numberingupwards in the clockwise direction. Gate driver IC 312 may comprise, forexample, the IRD2010 driver IC available from International Rectifier.Substrate 300 further includes solder bars 354 a, 354 b, 356 a, 356 b,358 a, 358 b, 360 a, 360 b, 344, 362, 346, 364, 348, 366, 322 a, 322 b,and 350, which may correspondingly receive the SFMs exposed byIII-nitride die 310 corresponding to power device 200 when flipped ontosubstrate 300. The substrate may help position the SFMs by providing,for example, matched openings with 25 microns of empty padding on allsides for solder fill.

The grey portions of substrate 300 below the solder bars are coppertraces, which may be preferably 1 to 2 oz per square inch or thicker toimprove conductivity and reduce resistance. Several vias such as via 352are also shown throughout substrate 300, and may preferably comprisewalls of at least 40 microns to improve conductivity and reduceresistance. The vias may route to external circuit connections such asto a printed circuit board. Although the vias are depicted in FIG. 3 asthe same size, they may actually differ in size.

Solder bars 362, 364, and 366 will make contact to the SFMs that areconnected to the drain of control PET 230, which receives Vin from gatedriver IC 312. Solder bars 344, 346, 348 and 350 will make contact tothe SFMs that are connected to the source of synch FET 240 or powerground (P-ground), distinguished from analog or signal ground. Whilesolder bars 344, 346, 348 and 350 may appear to be isolated, they areactually grounded together by vias to an unseen grounding layer. Solderbars 354 a, 354 b, 356 a, 356 b, 358 a, 358 b, 360 a and 360 b areconnected to a switch node 336 which connects the source of control FET230 to the drain of synch FET 240. Switch node 336 also functions as the“gate return” for control PET 230. Node 334 is connected to the gate ofcontrol PET 230, node 338 is connected to P-ground and also functions asthe “gate return” for synch FET 240, and node 340 is connected to thegate of synch FET 240.

Node 332 connects gate driver IC 312 to capacitor 314 e, which maycomprise a bootstrap or boost (BST) capacitor. Node 342 connects gatedriver IC 312 to capacitor 314 d, which may comprise a bypass capacitorfor filtering ripple noise. Capacitors 314 a and 314 b are also shown inFIG. 3, connected in parallel and receiving Vin from gate driver IC 312.As shown in FIG. 3, solder pads 370 a through 370 d and 372 a through372 d are positioned appropriately on substrate 300 to receivecapacitors 314 a through 314 d.

Referring now to FIG. 4, a cross-sectional view of an exemplarymulti-level III-nitride power device mounted on a substrate according toone embodiment of the invention is illustrated. As discussed above, thecross section of FIG. 4 is taken from cross section A-A of FIG. 3.HI-Nitride die 410 is flipped, with the exposed SFM resting directly onsolder bars 404 and 406. Switch node 336 of FIG. 3 is shorted throughcopper trace 482, which connects the drain of synch FET 240 resting onsolder bar 404 and the source of control FET 230 resting on solder bar406.

As can be seen in FIG. 4, using solder bars 404 and 406 running parallelwith the SFM of III-nitride die 410 rather than using conventionalsolder balls or bumps provides greater surface area for conduction andheat dissipation, advantageously reducing Rdson. Furthermore, sincesolder bars 404 and 406 are continuous in lateral directions, currentcan be routed laterally through solder bars 404 and 406, furtherincreasing conduction. In contrast, if conventional solder balls orbumps were utilized, then current could not travel in the lateraldirection since the balls would introduce non-conductive gaps, requiringcurrent to be rerouted through copper trace 482 or III-nitride die 410.

Referring now to FIG. 5, a circuit diagram integrating an exemplarymulti-level III-nitride power device according to one embodiment of theinvention is illustrated. Control FET 510 a, labeled Q1, may correspondto control FET 230 of FIG. 2. Synch FET 510 b, labeled Q2, maycorrespond to synch FET 240 of FIG. 2. Capacitor 512 a, labeled C1, maycorrespond to the parallel-connected capacitors 314 a and 314 b in FIG.3. Capacitor 512 c, labeled C2, may correspond to capacitor 314 c inFIG. 3. Capacitor 512 d, labeled C3, may correspond to capacitor 314 din FIG. 3. Gate driver IC 514, shown as an IRD2010 driver IC byInternational Rectifier, may correspond to gate driver IC 312 shown inFIG. 3.

As discussed above, the SFMs fabricated and integrated on the die of aIII-nitride power device and connected to solder bars on the packagesubstrate together minimize Rdson and therefore improve conductivity inthe switch node (SW) between FETs 510 a (Q1) and 510 b (Q2), and enablepower circuit 500 shown in FIG. 5, which can be, for example, a DC to DCbuck converter (also referred to generally as a “DC-DC conversioncircuit” in the present application), to operate more efficiently. Thus,as discussed above, in the embodiments of FIGS. 1 through 5, theinvention achieves a III-nitride power device and package therefor thatresult in, among other things, enhanced thermal and conductivityperformance including a reduced effective Rdson for the powertransistors used therein.

From the above description of the invention it is manifest that varioustechniques can be used for implementing the concepts of the presentinvention without departing from its scope. Moreover, while theinvention has been described with specific reference to certainembodiments, a person of ordinary skill in the art would appreciate thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. For example, conducting andnon-conducting cells can have varying elements and configurations whilestill embodying the spirit of the present invention. Thus, the describedembodiments are to be considered in all respects as illustrative and notrestrictive. It should also be understood that the invention is notlimited to the particular embodiments described herein but is capable ofmany rearrangements, modifications, and substitutions without departingfrom the scope of the invention.

The invention claimed is:
 1. A III-nitride power semiconductor deviceincluding a first transistor comprising: a first III-nitride layer; asecond III-nitride layer forming a heterojunction with said firstIII-nitride layer; a gate nitride, a source, and a drain over saidsecond III-nitride layer; a top metal layer and a bottom metal layer onsaid second III-nitride layer, said bottom metal layer including asource contact over said source, a drain contact over said drain, and agate contact over said gate nitride; a plurality of vias connecting saidsource contact, said drain contact, and said gate contact to said topmetal layer; a solderable front metal over said top metal layer.
 2. TheIII-nitride power semiconductor device of claim 1, wherein saidsolderable front metal comprises spaced elongated digits configured forexternal circuit connection.
 3. The III-nitride power semiconductordevice of claim 2, wherein said spaced elongated digits alternatelyexpose said source contact and said drain contact.
 4. The III-nitridepower semiconductor device of claim 1, wherein said solderable frontmetal comprises solder bars.
 5. The III-nitride power semiconductordevice of claim 4, wherein said solderable front metal is soldered withsaid solder bars to conductive traces of a circuit board for externalcircuit connection.
 6. The III-nitride power semiconductor device ofclaim 4, wherein said solderable front metal is soldered with saidsolder bars to conductive traces of a circuit board for implementing aDC-DC conversion circuit.
 7. The III-nitride power semiconductor deviceof claim 1, wherein said first transistor is a high electron mobilitytransistor (HEMT).
 8. The III-nitride power semiconductor device ofclaim 1, further comprising a second transistor, wherein said solderablefront metal of said first transistor and a solderable front metal ofsaid second transistor expose alternating interdigitated source anddrain contacts.
 9. The III-nitride power semiconductor device of claim1, further comprising a passivation on said top metal layer, saidpassivation surrounding said solderable front metal.
 10. The III-nitridepower semiconductor device of claim 1, further comprising a passivationon said top metal layer, said passivation selected from the groupconsisting of epoxy, polyamide, and silicon oxide.
 11. The III-nitridepower semiconductor device of claim 1, wherein said first HI-nitridelayer comprises GaN and wherein said second III-nitride layer comprisesAlGaN.
 12. The III-nitride power semiconductor device of claim 1,wherein said solderable front metal comprises Titanium-Nickel-Silver(TiNiAg) tri-metal.
 13. The III-nitride power semiconductor device ofclaim 1, wherein said first III-nitride layer is on a silicon substrate.14. A III-nitride power semiconductor device comprising: a first highelectron mobility transistor (HEMT) having multiple interconnect metallayer's, said first HEMT including a solderable front metal on a topmetal layer, said solderable front metal exposing a gate contact, asource contact, and a drain contact of said first HEMT, wherein saidsolderable front metal is configured for external circuit connection.15. The III-nitride power semiconductor device of claim 14, furthercomprising a second HEMT, wherein said solderable front metal of saidfirst HEMT and a solderable front metal of said second HEMT exposealternating interdigitated source and drain contacts.
 16. TheIII-nitride power semiconductor device of claim 14, further comprising apassivation on said top metal layer, said passivation selected from thegroup consisting of epoxy, polyamide, and silicon oxide.
 17. TheIII-nitride power semiconductor device of claim 14, wherein said firstHEMT is an AlGaN/GaN HEMT.
 18. The III-nitride power semiconductordevice of claim 14, wherein said solderable front metal comprisesTitanium-Nickel-Silver (TiNiAg) tri-metal.
 19. The III-nitride powersemiconductor device of claim 14, wherein said first HEMT is disposed ona silicon substrate.
 20. The III-nitride power semiconductor device ofclaim 14, wherein said solderable front metal is soldered to conductivetraces of a circuit board for implementing a DC-DC conversion circuit.